The present invention relates to transformer differential relays in general, and more particularly, to a transformer differential relay which includes apparatus for speeding up the detection of a power transformer internal fault condition to initiate breaker action which interrupts current through the transformer.
Present transformer differential relays utilize a percentage type differential measuring unit for detecting potential internal fault conditions in a power transformer of a power system and for initiating breaker action to isolate the transformer from its current paths in the power system. Under certain situations, like when the transformer is initially energized or when a collapsed voltage is recovered following clearing of a certain external fault, the resulting magnetizing initial or recovery inrush current conditions appear to the differential relay as an internal fault condition and may produce false breaker operation. Conventional transformer differential relays include a second measuring unit to detect an inrush current condition by measuring the content of a selected harmonic of the power system frequency and to block or restrain breaker operation by the differential unit upon such detection. This discrimination between inrush current and internal fault current conditions is possible because the present power transformer characteristics cause severe distortion of the inrush current waveform rendering it rich in second harmonic content which may be on the order of 15% of the fundamental signal content. The secondary measuring units in the differential relays are commonly referred to as harmonic restraint units.
In order to detect an inrush current condition, the harmonic restraint unit must collect enough of the differential-current signal to determine the ratio of the line frequency component to the second harmonic component of the waveform. Generally, this requires on the order of a cycle of the powerline waveform or more of signal data regardless of the method used to find the inrush current ratio. Accordingly, the response time of the differential relay is delayed by the processing time of determining whether or not an inrush current condition exists before it may issue breaker operation.
If these inrush current conditions are ignored, it is entirely feasible to implement an electronic differential relay which can trip in 3-5 ms for internal faults basing the differential protection solely on the instantaneous vector sum of measured current signals representive of the currents of a common power system phase of the transformer; no processing of the measured current signal waveforms is needed to detect an inrush condition, thus no delay is incurred. However, such a relay will also cause unwanted breaker action for false detection of internal faults under inrush current conditions.
One approach to speeding up the operation of the differential relay without incurring false internal fault breaker operation is disclosed in the U.S. Pat. No. 4,402,028, issued Aug. 30, 1983 to Eric A. Udren, entitled "Protective Relay Methods and Apparatus". In the Udren relay, harmonic restraint is preempted by voltage supervision, allowing the differential current unit thereof to make trip decisions within 3-5 milliseconds after the internal transformer fault is induced. The Udren relay operates using the principal that the voltage in the transformer zone increases rapidly upon inrush current conditions. The harmonic restraint function is invoked when the rate of change of voltage increase in the transformer zone exceeds a predetermined rate value. Under the conditions of a decreasing or steady voltage in the transformer zone, the harmonic restraint operation is preempted by the voltage supervision allowing the aforementioned response time to initiate breaker operation to an induced internal transformer fault. The primary drawback of the Udren relay is that, for the most part, power system transformer relays do not have voltage measuring devices available, thus the application of the Udren relay is limited to where voltage measurement and instrumentation is available or where potential transformers and wiring connections are provided at the transformer.
On the other hand, current measuring devices are provided at the transformer and include wiring connections to the site of the protective relays therefor. Thus, it would be completely desirable to have a current supervision approach for speeding up the response time of a transformer differential relay while inhibiting breaker operation due to unwanted false internal fault conditions such as initial and recovery inrush current conditions, for example. A transformer differential relay with current supervision for response speed-up purposes can utilize the current measuring devices in place about the transformer to be protected without the necessity of adding additional measuring devices and the wiring connections therefor.